It is known that a non-single-crystal deposited film composed of amorphous or polycrystalline silicon material which is prepared by a glow discharge in silane gas (SiH.sub.4) has good photoconductive characteristics because dangling bonds of the silicon are terminated with hydrogen atoms. It is also known that it is possible to form a deposited film having p-type or n-type conductivity by doping the foregoing deposited film of an i-type conductivity with a trivalent or pentavalent element. Such deposited film can be formed at a relatively low temperature, and it is possible for such deposited film to be of a large area. Because of this, the foregoing non-single-crystal film is advantageous in preparing an opto-electric conversion element to be used for a solar cell.
However, for such silicon containing non-single-crystal film prepared by means of the glow discharge process, its band gap energy measured based on light absorption is in the range of about 1.7 eV to 1.8 eV, which is not always equivalent to the wavelength of solar radiation, and because of this, there is a limit on the photoelectric conversion efficiency of such solar cells prepared using such silicon containing non-single-crystal films.
In order to obtain a desired opto electric conversion element having a high photoelectric conversion efficiency, it is necessary to provide a deposited film of a desirably small band gap energy. In view of this, there has been made a proposal to use a deposited film containing silicon atoms and germanium atoms as the constituent atoms for a deposited film whose band gap energy is smaller than that of the foregoing silicon containing non-single-crystal film, which is prepared by a glow discharge in a mixture of SiH.sub.4 gas and GeH.sub.4 gas, as the i-type layer in the opto-electric conversion element.
However, it is generally recognized that the resulting deposited film containing silicon atoms and germanium atoms prepared by means of the glow discharge process often is insufficient not only in photoconductive characteristics but also in photoconversion efficiency even though it is satisfactory from the viewpoint of an band gap energy.
In addition, in the case of continuously forming all the constituent layers of the opto-electric conversion element, i.e. the p-type layer, i-type layer, and n-type layer by means of the glow discharge process, there will occur problems, for example, in forming the i-type layer after the formation of the p-type layer because excited species, particularly ions, generated during its formation process tend to sputter the previously formed p-type layer and cause a p-type dopant such as boron present therein to contaminate the i-type layer to be formed. There will occur similar problems also in the case of forming the i-type layer after the formation of the n-type layer in that an n-type dopant such as phosphorus present in the n-type layer will, because of sputtering by the foregoing ions, contaminate the i-type layer to be formed.